1. Field of the Invention
The present invention relates to a glass-impregnated fiber-reinforced ceramic suitable for use in a hot oxidizing atmosphere, and a method of manufacturing the same.
2. Description of the Related Art
Ceramics have become watched as lightweight, highly eat-resistant structural materials in recent years owing to their excellent heat resistance superior to that of metals, and their high specific strength. However, there are still problems in using ceramics as structural materials in respect of reliability because ceramics have low toughness as compared with that of metals. Efforts have been made for the development of fiber-reinforced ceramic composite materials having high toughness as well as the excellent characteristics of ceramic materials.
Generally, the toughness of a fiber-reinforced ceramic (hereinafter referred to sometimes as "FRC") is most greatly dependent on the bonding strength of the interface between the fibers and the matrix of the FRC. The toughness is high if the bonding strength of the interface between the fibers and the matrix is low, and is low if the bonding strength of the interface between the fibers and the matrix is high. Such a characteristic of toughness is considered to be due to a fact that energy is consumed in extracting the fibers from the matrix if the bonding strength of the interface between the fibers and the matrix is low. Accordingly, when producing an FRC having a high toughness, the bonding strength of the interface between the fibers and the matrix of the FRC is reduced properly by, for example, coating the fibers.
The fiber-reinforced ceramic has a ceramic matrix, and inorganic fibers, i.e., reinforcing material, embedded in the ceramic matrix, and the ceramic matrix and the inorganic fibers are chosen out of various materials. The following are representative FRC manufacturing methods.
(a) Method which molds a mixture of fibers and a ceramic precursor, namely, a substance which can be converted into a ceramic material by thermal decomposition, in a molding and fires the molding.
(b) Method which forms a structure only from fibers, impregnates the structure with a ceramic precursor, and fires the structure impregnated with the ceramic precursor (Japanese patent application unexamined laid open No. 8-157271).
(c) Method which forms a structure only from fibers, impregnates the structure with a molten ceramic material, and fires the structure impregnated with the molten ceramic material.
Fiber-reinforced ceramics having excellent heat resistance, high specific strength and high toughness as mentioned above are used widely for various purposes. However, the following problems arise when conventional FRCs are used in a hot oxidizing atmosphere.
Referring to FIG. 5 showing a fiber-reinforced ceramic 1 manufactured by a conventional method in a typical sectional view, the fiber-reinforced ceramic 1 has a ceramic matrix 4, and inorganic fibers 3 embedded in the ceramic matrix 4, and the ceramic matrix 4 has voids 5 which develop unavoidably during a manufacturing process. If the inorganic fibers 3 are, for example, silicon carbide fibers, the ceramic matrix 4 is silicon carbide, and the FRC 1 is heated at 1200.degree. C. in the atmosphere, oxygen permeates the FRC through the voids 5, reaches the interfaces between the silicon carbide fibers and the silicon carbide matrix, and oxidizes the silicon carbide fibers and the silicon carbide matrix to produce SiO.sub.2 glass in the interfaces. The SiO.sub.2 glass formed in the interfaces bonds the silicon carbide fibers and the silicon carbide matrix firmly together, which reduces the toughness of the FRC 1 greatly. Therefore, the strength at elevated temperature of the FRC 1 in an oxidizing atmosphere or the strength of the same after the FRC 1 has been exposed to an oxidizing atmosphere is reduced greatly. Such a phenomenon is a problem common to FRC 1 consisting of easily oxidizable components. The notch effect of the voids 5 reduces the strength of the conventional FRC 1.
The conventional art method proposed to solve the foregoing problem in Japanese patent application unexamined laid open No. 8-157271 coats the surface of an FRC with glass. This method improves the oxidation resistance and the room temperature strength of the FRC by the sealing effect of the glass. However, the properties of the FRC manufactured by this method are still below the level of properties required of structural materials for use in a hot oxidizing atmosphere.